1. Technical Field
The present invention relates to a membrane electrode assembly for polymer electrolyte fuel cells, and in particular, relates to a technology which can restrain leaching of a catalyst material when there is a shortage of fuel gas, such as hydrogen, in the anode side.
2. Background Art
A polymer electrolyte fuel cell (hereinafter simply referred to as “fuel cell”) is composed by laminating separators at both sides of a flat electrode structure. The electrode structure is a stacked element having a polymer electrolyte membrane held between a positive side electrode catalyst layer and a negative side electrode catalyst layer, with a gas diffusion layer laminated outside of each electrode catalytic layer. In such a fuel cell, for example, when hydrogen gas is supplied in a gas passage of the separator disposed at the negative electrode side, and an oxidizing gas is supplied in a gas passage of the separator disposed at the positive electrode side, an electrochemical reaction occurs, generating an electric current. During operation of the fuel cell, the gas diffusion layer transmits the electrons generated by electrochemical reaction between the electrode catalytic layer and the separator, and diffuses the fuel gas and the oxidizing gas at the same time. The negative side electrode catalytic layer induces a chemical reaction in the fuel gas to generate protons (H+) and electrons, and the positive side electrode catalytic layer generates water from oxygen, protons and electrons, and the electrolyte membrane transmits protons by ionic conduction. As a result, electric power is provided from the positive and negative electrode catalytic layers.
As an electrode catalytic layer, a mixture of carbon particles carrying a catalyst material such as a rare metal and an electrolyte consisting of an ion conducting polymer, is well known. Pt alone or Pt with other metals is known as the catalyst material. In particular, a Pt—Ru alloy is used in a fuel electrode to prevent Pt from being poisoned by impurities such as carbon monoxide included in hydrogen gas which is used as a fuel gas.
A fuel cell is used in stacks in which dozens to hundreds of membrane electrode assemblies are laminated via separators. In a situation in which such a fuel cell is operated, it is known that there may be a temporary shortage of fuel gas, thereby generating reverse voltage in the entirety or a part of the anode due to the delay of fuel gas supply when the output changes extremely, or water-discharging ability of the membrane electrode assembly is reduced. If the reverse voltage is generated, electrolysis of water is promoted, and further more, it is known that carbon particles are corroded. This is considered to occur as following reactions.H2O→½O2+2H++2e−½C+H2O→½CO2+2H++2e−
WO01/15247, WO01/15249, WO01/15254, and WO01/15255, methods to prevent carbon particles from being corroded, are disclosed.
However, the deterioration of generation efficiency cannot be prevented sufficiently by these methods.